1. Field of the Invention
The field of this invention relates generally to direct conversion receivers, and, more specifically, to subharmonic frequency translators for use in such receivers, and preprocessors for improving the switching characteristics of the LO input to such frequency translators.
2. Background
Conventional receivers downconvert a radio frequency (RF) signal to baseband frequencies in two steps. In the first step, the signal is downconverted to intermediate frequencies (IF), and in the second step, the signal is downconverted to baseband frequencies. A conventional receiver is illustrated in FIG. 1. An RF signal 1 having a carrier frequency FRF is applied to RF input port 3 of mixer 2. A signal 10 having a frequency Fx less or greater than FRF originating from local oscillator (LO) 9 is passed through bandpass filter 15 and then through a low noise amplifier (LNA) (not shown). The resulting signal is then applied to the LO input port 4 of mixer 2. The mixer 3 mixes the two signals and provides an output signal at output port 5. The output signal has two principal frequency components: one at the frequency FRF−Fx (or Fx−FRF in the case in which Fx is greater than FRF), the so-called intermediate or IF frequency FIF, and the other at the frequency FRF+Fx.
The signal is passed through IF filter 6 which substantially attenuates the component at the frequency FRF+Fx, thus leaving the intermediate frequency component. The output of the filter including this intermediate frequency component is identified with numeral 7.
This signal is provided to the signal input port of mixer 8. At the same time, a signal 12 at the same intermediate frequency and originating from local oscillator 12 is applied to the LO input port of mixer 8. Mixer 8 mixes the signals provided at its two inputs, and produces an output signal having two principal frequency components: one at the frequency 2FIF, and the other at zero or baseband frequencies FBB. The output of the mixer 8 is passed through baseband filter 14 which substantially attenuates the component at the frequency 2FIF, thus leaving the component at baseband frequencies. The output of the filter is identified in the figure with numeral 13.
Direct conversion receivers downconvert an RF signal to baseband frequencies in a single step. Typically, a mixer mixes an RF signal with an LO signal at the same frequency as the carrier of the RF signal. The mixer produces two primary frequency components in the output signal: one at the difference frequency FRF−FLO, and the other at the frequency FRF+FLO. Since the LO signal is at the same frequency at the RF signal, the first such component is at baseband frequencies, and the second such component is at high frequencies.
The output of the mixer is passed through a baseband filter, which substantially attenuates the high frequency component of the output, leaving the baseband component. Compared to the conventional receiver of FIG. 1, direct conversion receivers eliminate components such as the IF filter 6, one of the mixers, and one of the local oscillators. The elimination of the IF filter is particularly advantageous because such filters tend to be bulky, expensive, and not implementable on-chip.
However, direct conversion receivers are typically limited in their sensitivity because of leakage from the LO port to the RF port, or leakage of large RF blockers from the RF port to the LO port, both of which can result in self-mixing, and introduction of a large unwanted DC component in the output signal.
Subharmonic mixers are mixers in which the LO frequency is a subharmonic of the RF frequency. Subharmonic mixers allow generation of lower frequency LO signals, which eases synthesizer and voltage controlled oscillator (VCO) design. They also provide the potential for frequency isolation between the LO and RF signals.
Unfortunately, most subharmonic mixers have relatively low conversion gain and high noise as compared to standard mixers. They are also plagued by the presence, on an internal node or pin, of LO harmonics which, because they are at the mixing frequency, can still self-mix to DC. Some also require bulky transformers that limit or prevent on-chip implementation. Most also have substantially non-linear RF transfer functions.
The Gilbert mixer is one type of mixer which allows for conversion gain. However, conventional Gilbert mixers include a mixer core which is unable to accommodate a LO frequency which is a subharmonic of the RF frequency.
More specifically, the standard Gilbert mixer is driven by an LO signal having two components which are 180° are of phase with respect to one another. The components are clipped in order to increase the transition times thereof, thereby improving noise performance and achieving a higher conversion gain. However, such techniques cannot be generalized to the subharmonic mixer case.
Therefore, there is a need for a direct conversion receiver with increased gain, noise performance, and sensitivity compared to the prior art.
There is also a need for a subharmonic mixer which is capable of on-chip implementation, has conversion gain, noise figure, and linearity characteristics comparable to or exceeding those of conventional mixers, and does not generate on an internal pin or node LO harmonics at the mixing frequency.
There is also a need for a preprocessor which improves the switching characteristics of successive phase-split LO-inputs.